In packaging industry, the properties needed for different applications may be diverse. The package may be required to form airtight, aseptic and mechanically durable sealing to protect the packaged product throughout its route from factory to market. This is essential for food. On the other hand, the same package should be easy to handle and open by the end consumers of the goods. To fulfil every requirement, compositions with several layers of the same or different materials are often used. Different layers serve different purposes, e.g. visual, barrier, carrier, tearing, sealing, etc. For manufacturing such multilayer compositions, typical processes are coating, laminating, extrusion coating and coextrusion.
Coating a substrate, i.e. a paper or board web, with a coating agent, has been typical refinement in production of high-quality surfaces. The coating process is performed either in connection with the paper-making machine, as an on-line process, or as a separate off-line process. In an on-line process, the continuous web having been formed in the paper-making machine runs directly to the coating machine, and the web is wound only after the coating process steps. In off-line coating, the web is wound after the paper-making machine and this web is coated in a separate coating machine by seaming a new roll after each web unwound from the preceding roll.
There is a range of different options available for the coating unit: air-knife, blade coaters, size press coaters, spray coaters, curtain coaters, electrostatic coating methods etc. The common feature for all these coating units is application of an aqueous coating paste over the entire width of the dry web, followed by drying of the coating paste and the partly wetted web by means of driers, such as infrared radiators, air blow driers or cylinder driers. The coating paste typically has a solids content of the order of 40 to 70%, while pigmented formulations used in flims press or size press treatments runs with lower solid content. Typical coating formulations in traditional coating include e.g. kaolin and calcium carbonate, minerals, binders, rheology modifiers and additives. The coating process may be repeated a number of times to achieve a surface with excellent performance. Such a combination may comprise e.g. coating of both sides of the web, first with a size press coater and subsequently coating of both sides with a blade coater. Calendaring usually follows coating to achieve appropriate gloss and smoothness for the surface. Then the web is formed as a “machine roll”, which, in turn, is divided in a winder into rolls with smaller width and web length adapted to a printing machine.
When considering substrates for printing, the requirements for coating layers are related to uniformity, smoothness, gloss, colour, opacity, surface energy, retention, colour adsorption, etc. In case of coating formulations for board used to package food products and like, the FDA approval and consent to odour and taste requirements are crucial, which often eliminates the use of wide-ranging class of functional chemicals. In case of print quality related to wetting and adhesion, the most common way to control the interactions are via modification of the surface energy. Traditional coating methods and agents, the aim has traditionally been to improve adhesion. Said methods can be surface treatment, mechanical roughening, removing weak boundary layers, minimising stresses, using adhesion promoters, using suitable acid-base interactions, as well as providing favourable thermodynamics and using wetting. Typical treatment techniques include the use of chemicals such as primers and solvents, the use of heat and flame, mechanical methods, plasma, corona treatment and radiation. Each technique can improve adhesion via different influences. Desired effects include promoting adhesion between the substrate and the coating by increasing the free energy (wettability) of the surfaces, inducing chemical reaction between them, and removing bond weakening impurities from them.
In case of too strong adhesion between dissimilar or similar layered materials various lubricants and on the other hand, powders such as talc can be introduced to reduce surface energy or contact between the materials. Contacts may be between solid and solid or between solid and liquid. Although these substances facilitate the processing, their presence in or on the end product surfaces may be undesirable, even prohibited, as is the case with food. Problems within traditional coating methods arise from different requirements for surfaces during different phases of the life cycle of the package. During production, on the production line, the units should flow liquidly, but during transport, too slick surfaces may cause drifting of the load with collisions and breaking of the packages.